Over the past couple of years much attention has been given to the plight of the monarch butterfly and the need to create habitat by planting milkweed (the only food for the caterpillars) and flowering plants to provide nectar (food) for the adult butterflies. When purchasing plants, make sure that the milkweed plants have not been treated with systemic neonicotoids. According to LSU Agricultural agent, Dr. Joe Willis, the neonicotoids will dilute as the plants grow but only a very small amount of it will kill the larva of the monarch butterfly. It is best that any plants purchased be neonicotoid free since that chemical is suspected to be related to colony collapse disorder in bees.
Another issue is that the plant being sold is a variety of tropical milkweed, a non-native. The non-native milkweeds lead to higher parasitic infections among monarchs; especially in the southern states where it grows all winter long and monarchs feed on it and continue to reproduce. To learn more about tropical milkweed and its implications for monarchs, read this research.
The best milkweed varieties for monarchs are native ones. Suggested ones for Wisconsin are:
Common Milkweed (Asclepias syriaca)
Swamp Milkweed (Asclepias incarnata)
Butterfly Weed (Asclepias tuberosa)
The National Wildlife Federation lists twelve native milkweeds for options in monarch butterfly plantings.